1. Field of the Invention
The invention relates to a method for forming a thin film resistor.
2. Description of the Related Art
In the art of microelectronics fabrications, such as integrated circuit microelectronics fabrications and hybrid circuit microelectronics fabrications, thin film resistors are used as passive electrical circuit elements.
When thin film resistors are used in hybrid circuit microelectronics fabrications, a typical method for forming the resistors comprises the steps as follows: First, a layer of thin film resistive material and a layer of conductive material are formed, in this order, over an insulator substrate such as a glass insulator substrate or a ceramic insulator substrate. Next, the conductive material and then the resistive material are patterned by photolithography to form a patterned conductive layer and a patterned resistive layer. Next, the resistance of the patterned resistive layer is determined through laser trimming and the insulator substrate is then parted to form discrete thin film resistors.
The above-mentioned method of the prior art has the following disadvantages. Since the patterned conductive layer and the patterned resistive layer are formed by photolithography, and costs of photolithographic apparatus and materials are high, the fabrication costs of discrete thin film resistors are high. In the prior art method for forming the patterned conductive layer and the patterned resistive layer by photolithography, an insulator substrate of enhanced surface flatness and finish is typically required in a spin-coating process of a photoresist layer, in order to ensure the photoresist layer having an uniform thickness and provide an adequate registration of the insulator substrate within photolithographic apparatus. Generally, an aluminum oxide blank substrate with a purity of 99.6% is required, and it needs to be polished in order to enhance its surface flatness and finish. Compared with a conventional aluminum oxide substrate already provided with scribe lines, the aluminum oxide blank substrate cannot be cut simply along the scribe lines at a low cost but must be cut using laser, and thus further increases the fabrication costs of discrete thin film resistors.
Therefore, it is the object of the invention to provide a method for forming a thin film resistor solving all the above-mentioned problems.
The method for forming a thin film resistor in accordance with the invention comprises the following steps: providing an insulator substrate; forming a patterned conductive layer over the insulator substrate by a non-photolithographic method; forming a thin film resistive layer on the patterned conductive layer and the insulator substrate; applying photoresist by a known dry film process or a spray coating process in place of a spin coating process and then patterning the thin film resistive layer by photolithography. Since the dry film process and the spray coating process can be applied to an uneven surface of substrate, thin film resistors can be formed on a conventional aluminum oxide substrate already provided with scribe lines and can be separated by simply cutting along the scribe lines with low costs. Thus, the costs of raw materials and the manufacturing costs can be lowered. In this case, the insulator substrate can be a glass insulator substrate or a ceramic insulator substrate. A leveling layer can be formed on the insulator substrate before forming the patterned conductive layer over the insulator substrate.
The above-mentioned non-photolithographic method can be a screen printing method. The patterned conductive layer can be made from a material comprising silver, silver alloys, gold, gold alloys, copper, copper alloys, palladium, palladium alloys, nickel, or nickel alloys. In addition, the thin film resistive layer can be made from a material comprising tantalum nitride resistive materials, tantalum silicide resistive materials, tantalum-chromium alloy resistive materials, nickel-chromium alloy resistive materials, chromium silicide resistive materials, or higher order alloys of the aforementioned resistive materials. The thin film resistive layer can be formed by a method selected from the group consisting of thermally assisted evaporation methods, electron beam assisted evaporation methods, physical vapor deposition methods, chemical vapor deposition methods, or plasma enhanced chemical vapor deposition methods.
Laser trimming can adjust the resistance of the patterned thin film resistive layer. In addition, the resistance of the thin film resistive layer can be adjusted during the photolithographic patterning of the thin film resistive layer.
Using the method of the invention, since a non-photolithographic method is used in place of a conventional photolithographic method to form the thin film resistive layer, the fabrication costs of a thin film resistor is lowered. Furthermore, an aluminum oxide substrate with a purity of 99.6%, used in conventional method, is not required in the invention. Instead, an aluminum oxide substrate with a purity of 96% can be used, which greatly lowers the fabrication costs of the discrete thin film resistors.
The above and other objects, advantages, and features of the invention will become apparent from the following description with reference to the accompanying drawings.